RELATIONSHIP BETWEEN SLIPPING FRICTION OF PREPREG STACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-SHAPED THERMOSETTING COMPOSITE LAMINATES

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1 THE 19 TH INTERNATIONAL CONFERENCE ON COMPOS ITE MATERIALS RELATIONSHIP BETWEEN SLIPPING FRICTION OF PREPREG STACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-SHAPED THERMOSETTING COMPOSITE LAMINATES J. Sun*, Y.Z. Gu, M. Li, Y.X. Li, Z.G. Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing, China * Corresponding author (suncrystal@mse.buaa.edu.cn) Keywords: Thermosetting composite; Prepreg; Buckling; Internal friction; Diaphragm forming 1 General Introduction Using prepreg materials, traditional hand lay-up process is often adopted to manufacture composite components of aircrafts. With increasing use of composites in airplanes, this labor-intensive and time-consuming process is being challenged. To enhance product quality and increase productivity, automated tape laying (ATL) technique is developed, which lays up prepreg stacks with high efficiency and is suitable for producing parts with flat or large curvature radius surfaces [1 3]. However, ATL is unable to directly lay up more complex composite parts with angle-shaped structure, such as L-shaped stringers and C-beams. For this kind of components, a flat prepreg laminate is usually formed by ATL and then deformed into the designed shape with abrupt contours using hot diaphragm forming process. The formed prepreg structure is finally cured in an autoclave. Therefore, the hot diaphragm forming process realizes laying up complex structures through automated technology. The hot diaphragm forming process for thermosetting composite materials is a derivation of diaphragm forming of thermoplastic sheets. During the hot diaphragm forming process, a preformed flat lay-up laminate is typically placed between two layers of high ductility diaphragm films and heated by infrared heating. The prepreg structure is then stretched over mold to obtain the desired shape by application of vacuum pressure. This process is designed to allow the individual plies to slip against each other and quickly produce parts with designed angle-shapes and consistent qualities. The technology has already been successfully applied to the production of large composite structures, such as long truss of Boeing 777 and front wing spar of A400M [4, 5]. In this article, C-shaped carbon fiber / epoxy prepreg preforms were produced using a designed hot diaphragm forming device under different processing temperatures and vacuum applying rates for two kinds of prepreg. The manufacturing defects after diaphragm forming process were evaluated, including fiber wrinkling and voids. Furthermore, interlaminar slipping friction of the prepreg stacks were experimentally analyzed and compared to the quality of C-shaped laminates formed under corresponding processing conditions. Based on these results of experiments, the relationship of the prepreg properties and the forming qualities of the formed C-shaped prepreg laminates was discussed. These results are essential to optimize process parameters in hot diaphragm forming and produce prepreg preform without wrinkles. 2. Experiment 2.1 Materials The prepreg materials used in the study were unidirectional carbon fiber/epoxy resin prepreg (Guangwei Co.) and unidirectional carbon fiber/epoxy resin X850 prepreg (Cytec Co.). The resin weight contents of prepreg and X850 prepreg are 31.7 and 35.5, respectively. In hot diaphragm forming process, the diaphragm material is SL850 (Airtech Co.) with breaking elongation of 450 %, tensile strength of 82 N / mm 2 and maximum operational temperature of 204 o C. For better understanding of the investigated material system, rheological measurements of these two kinds of epoxy resin used in the prepreg stacks were conducted at a heating rate of 10 /min using a rheometer (Gemini, Bohlin Instruments). The dissolution method was used to obtain the resin of prepreg.

2 2.2 Hot Diaphragm Forming Process A hot diaphragm forming device allowing a flat prepreg laminate to be formed into a C-shaped laminate was designed, as shown in Fig. 1. The apparatus has a heating system and an operating vacuum system, which can control the forming temperature and the vacuum level. To achieve desired forming temperature, the heating power of the lamps, the distance between the lamps as well as the distance between the lamps and the stack surface were adjustable. A silicone rubber heating film was placed inside the C-shaped mold and controlled by an electrical heating facility. The vacuum pump was connected to the vacuum box and the space between the two membranes, as shown in Fig.1 (b). A gas flow meter and a control valve were applied to the system to give the control of the level and rate of vacuum. At the same time, a vacuum gauge showed the current vacuum level during the process. Before the diaphragm forming process, flat prepreg stacks were prepared in the size of mm at ambient temperature and the stacking sequence of the laminate was [45/-45] 8 cross-ply. During the diaphragm forming process, the mould was preheated accompanied by the flat stack under vacuum sealed state with the double diaphragms, and then the stack was heated by the infrared radiation lamps. After the stack was heated to the set temperature, vacuum in the vacuum box was applied and the flat preform progressively deformed into C- shaped prepreg laminate based on the geometry of the tool. More detailed description of the hot diaphragm forming process using this device can be found in our previous work [6]. 2.3 Characterization of the forming quality of prepreg preforms The C-shaped preforms made in this study have a flange length of 40 mm and a web length of 100 mm. The forming quality of the prepreg laminate was characterized by visual inspection for the surface morphology and micrograph observation of the cross-sections inside the preform. To preserve the states of the defects in preform and avoid the changes of the defects resulted from cutting and polishing operations during sample preparation, slow-curing method was employed for the preform. Firstly, the preform was slowly cured under 80 o C for 8 h in oven without any exerted pressure. This curing condition can make the curing degree reach 80% and ensure maintaining the original states of the defects during sample preparation. Then the cured preform was cut to get samples from different positions, as shown in Fig. 2. The cross sections of samples were wet ground with successively finer silicon carbide paper, from 600 to 2000 grit, and wet polished with chromium oxide. Finally, using an optical microscope (Olympus BX51M) the polished cross-sections were observed and void volume fractions were measured through the digital microscopy and image analysis to study the defects formed during the diaphragm forming process. 2.4 Measurement for frictional resistance of prepreg To characterize the slipping ability between individual prepreg plies, a testing device was designed to be mounted in a standard tensile testing machine (SANS Ltd. Co., 5KN), as shown in Fig. 3. The testing principle of this device is similar to the ones of the apparatus used by Martin et al. [7] and Ersoy et al. [8] to measure the frictional resistance of thermosetting prepreg. The specimens used for measuring interply friction were prepared by folding two mm prepreg strips which enclosed two mm plies. The short prepreg strips were pulled out from the long ones. The testing area was mm. Since both sides of the short ones underwent frictional effect, the load measured was theoretically equal to twice value of inter-ply frictional force. More details can be found in Ref. [6]. The temperature and pulling rate during the tests were set according to hot diaphragm forming process parameters. The pressure of 0.1MPa was applied to the test area with the application of four calibrated spring-screw sets. The samples were tested under 30 o C, 45 o C and 60 o C with a cross-head speed of 1 mm/min, while others were examined under 50 o C with cross-head speeds of 1 mm/min, 0.2 mm/min and 0.1 mm/min, respectively. A minimum of six specimens were tested for each condition. As depicted in Fig. 4, there is a slippage between plies on the ends of the preform, which results from the difference in the radius of curvature of the plies during the forming. Thus, the cross-head speed v used for the ply pull-out tests was calculated as follows: H ( n 1) t (1) where H is the length of the thickness variation region, n represents the layer number of the

3 RELATIONS HIP BETWEEN S LIPPING FRICTION OF PREPREG S TACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-S HAPED THERMOS ETTING COMPOS ITE LAMINATES preform, t is the time when the flat preform is deformed to the C-shaped one. In this way, the cross-head speed was determined based on the actual prepreg slipping velocity between adjacent plies with different vacuum pressure applying rate during hot diaphragm forming process [6]. The cross-head speeds of 1 mm/min, 0.2 mm/min and 0.1 mm/min in this paper correspond to the vacuum pressure applying rates of 2 L/s, 0.1 L/s, L/s, respectively. 3. Results and discussion 3.1 Effect of processing temperature on diaphragm forming process In hot diaphragm forming process, the forming temperature may have great influences on the resin viscosity and the ability of deformation for the prepreg stacks, so changes the forming process. In this section, the prepreg laminates with C-shaped structure were fabricated at 30, 45 and 60 with a vacuum applying rate of 2 L/s for two kinds of prepreg, to study the effect of forming temperature on the quality of prepreg laminates. The surface quality and the defects of the formed prepreg laminates were analyzed to evaluate the forming quality. Photographs of surface quality for two kinds of prepreg laminates are shown in Fig. 5 and the surface quality looks different at different processing temperature. It can be observed from these photos that for the prepreg, the preform produced at 30 o C has some wrinkles, while the preforms produced at 45 o C and 60 o C both have good surface quality without wrinkle. For the X850 prepreg, the preforms produced at 30 o C and 45 o C have wrinkles, while the preform produced at 60 o C has a good surface quality without wrinkle. Similar results also can be found from the micrographs of the crosssections inside the preforms, as shown in Fig. 6. This fiber buckling could affect the performance of cured composite component [9]. On the other hand, from Fig. 5 it can be seen that the void content decreases with the increasing temperature, indicating that higher temperature makes it easier to remove the entrapped air between plies during diaphragm forming process. In addition, the void content in the prepreg laminates is lower than that in the X850 prepreg laminates, which may be caused by the different surface morphology of the two prepregs [10]. 3.2 Effect of forming rate on diaphragm forming process Different vacuum applying rates can change the shaping rate of curved structure, and so change the interply slipping behavior of prepreg. In this paper, to study the influence of forming rate on the forming quality of hot diaphragm formed C-shaped preform, the C-shaped X850 prepreg laminates were produced under 50 o C with vacuum applying rates of 2 L/s, 0.1 L/s, L/s, respectively. Fig. 7 presents the surface states of preforms produced at 50 o C under different forming rates. The preform produced at 2 L/s shows some wrinkles at the corner. Visual inspection of the preform produced under L/s shows good surface quality and no signs of wrinkles. The results are confirmed by their optical micrographs. This result agrees with the observations for forming thermoplastic composites in Refs. [11, 12] that faster forming rates increase the severity of buckling. 3.3 Relationships between prepreg properties and forming qualities of prepreg laminates In hot diaphragm forming process, the interlaminar slip in prepreg stacks is one of the main mechanisms of deformation for producing angle-shaped structure. In order to further understand the formation of fiber wrinkles during the process, the friction behaviors between prepreg plies under the corresponding temperatures and rates were measured. Fig. 8 and Fig. 9 show the load-displacement behaviors of the representative specimens under different temperatures and forming rates, respectively. To simplify the results, the maximum value of interlaminar frictional force and the time of linear zone were extracted from the curves, as shown in Table 1 and Table 2. Compared to the forming qualities of prepreg laminates, it can be seen that no matter at different forming temperatures or at different forming rates, when the time of linear zone is bigger than the forming time of the C-shaped laminate, there are some wrinkles occurred on the surface of the perform, otherwise, there is no wrinkle. This is because that a straight curve means the plybonding state of prepreg plies, indicating that the slipping between individual layers is weak. When the time of linear zone is greater than the forming time of the C-shaped laminate, i.e.: during the entire molding process the prepreg plies are in a bonded state, and therefore there are wrinkles easily occurred on the surface of performs. When the time of linear zone is less than the forming time of the C shaped prepreg laminate, the adhesive state transforms into a sliding state for prepreg layers in 3

4 the molding process, the ability of relative movement improves and thus wrinkles disappear. Accordingly, the time of linear zone extracted from the curves of frictional resistance test can be defined as the biggest forming time of C-shaped prepreg laminate with wrinkles, which is helpful to control defects during the fabrication of C-shaped performs using the hot diaphragm forming process. 4. Conclusions In this study, C-shaped prepreg preforms produced in a home-made diaphragm forming laboratory facility were used to study the influences of process temperature and vacuum applying rate on the preform processing quality, including fiber wrinkling and voids. Furthermore, interlaminar slipping friction of the prepreg stacks were experimentally analyzed and compared to the quality of C-shaped laminates formed under corresponding processing conditions. The results show that temperature and vacuum applying rate both have great impacts on the surface quality of performs. Low temperature and high forming rate easily cause poor sliding of prepreg layers and large frictional force, which are the main reasons for fiber wrinkling at the corner of the preform. In addition, there is a close relationship between prepreg properties and forming qualities of parts, and a critical condition was achieved for optimizing processing conditions for the hot diaphragm formed C-shaped laminates, which provides an effective solution to defects control of traditional process. References [1] A. Mills Automation of carbon fiber preform manufacture for affordable aerospace applications. Composite Part A, Vol. 32, pp , [2] R.J. Crossley, P.J. Schubel and N.A. Warrior Experimental determination and control of prepreg tack for automated manufacture. Plastic Rubber Composite, Vol. 40, pp , [3] P. Debout, H. Chanal and E. Duc Tool path smoothing of a redundant machine: application to automated fiber placement. Computer Aid Design, Vol. 43, pp , [4] Z.E. Wu Hot Drape Forming of Composites. Aeronautical Manufacture Technology, Vol. S2, pp , [5] Y.L. Chen Application of Composite Forming Technique in Military Freighter A400M. Aeronautical Manufacture Technology, Vol. 10, pp 32-35, [6] J. Sun, Y.Z. Gu, M. Li, X.X. Ma, Z.G. Zhang Effect of forming temperature on the quality of hot diaphragm formed C-shaped thermosetting composite laminates. Journal of Reinforced Plastics and Composite, Vol. 31, pp , [7] C.J. Martin, J.C. Seferis and M.A. W ilhelm Frictional resistance of thermaset prepregs and its influence on honeycomb composite processing. Composite Part A, Vol. 27, pp , [8] N. Ersoy, K. Potter, M.R. Wisnom and M.J. Clegg An experimental method to study the frictional processes during composites manufacturing. Composite Part A, Vol. 36, No. 11, pp , [9] M.A. Keane, M.B. Mulhern and P.J. Mallon Investigation of the effects of varying the processing parameters in diaphragm forming of advanced thermoplastic composite laminates. Composite Manufacture, Vol. 6, No. 3-4, pp , [10] J. Sun, M. Li, Y.Z. Gu, D.M. Zhang, Y.X. Li and Z.G. Zhang Frictional resistance and surface structure of thermosetting prepreg with respect to hot diaphragm forming process. Journal of Composite Materials, DOI: / [11] G.B. McGuinness and C.M. ÓBrádaigh Effect of preform shape on buckling of quasi-isotropic thermoplastic composite laminates during sheet forming. Composite Manufacture, Vol. 6, No. 3-4, pp , [12] C.M. O'Brádaigh, R.B. Pipes and P.J. Mallon Issues in diaphragm forming of continuous fiber reinforced thermoplastic composites. Polymer Composites, Vol. 12, No. 4, pp , 1991.

5 RELATIONS HIP BETWEEN S LIPPING FRICTION OF PREPREG S TACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-S HAPED THERMOS ETTING COMPOS ITE LAMINATES (a) Photograph Fig. 1. Hot diaphragm forming device (b) Schematic presentation Fig. 2. Regions of the cured C-shaped laminate for micrograph observation (a) Photograph Fig. 3. Frictional resistance testing device (b) Test configuration 5

6 Fig.4 Frictional resistance testing device (a) 30 (b) 45

7 RELATIONS HIP BETWEEN S LIPPING FRICTION OF PREPREG S TACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-S HAPED THERMOS ETTING COMPOS ITE LAMINATES (c) 60 Fig. 5. Photographs of surface quality of prepreg laminates fabricated at different temperature (12500 prepreg on the left and X850 prepreg on the right) (a) 30 (b) 45 7

8 (c) 60 Fig. 6. Micrographs of cured C-shaped prepreg laminates fabricated at different temperatures (12500 prepreg on the left and X850 prepreg on the right) (a) 2 L/s (b) 0.1 L/s

9 RELATIONS HIP BETWEEN S LIPPING FRICTION OF PREPREG S TACKS AND FORMING QUALITY OF HOT DIAPHRAGM FORMED C-S HAPED THERMOS ETTING COMPOS ITE LAMINATES (c) L/s Fig. 7. Quality of C-shaped X850 prepreg laminates fabricated at different rates (Photographs on the left and Micrographs on the right) (a) prepreg (b) X850 prepreg Fig. 8. Frictional resistance of prepreg layers at different temperatures Fig. 9. Frictional resistance of X850 prepreg layers at different rates 9

10 Table 1. Comparisons of parameters of two kinds of prepreg at different temperatures Pepreg T ( ) Max. Frictional Forming Time of linear resistance (N) time (s) zone (s) X Table 2. Comparisons of parameters of X850 prepreg at different rates Rate (mm/min) Max. Frictional resistance (N) Forming time (s) Time of linear zone (s)